The present invention is useful in a process which we describe in patent applications filed contemporaneously herewith. Ser. No. 300,725 "Method for Simultaneous Peening and Smoothing" describes how workpieces are impacted with shot streams comprised of substantially uniform sized spherical particles having substantially uniform velocities. The workpieces which are being peened are simultaneously provided with residual compressive stresses and a smooth surface finish. Ser. No. 300,726 "Shot Peening Apparatus" describes equipment, including a holder for an airfoil which enables practice of the present invention. The disclosures of the foregoing applications are hereby incorporated by reference.
The present invention is especially useful for gas turbine airfoils. The high rotation speeds of gas turbines means that the surface finish of airfoils can greatly influence the efficiency of the machine. Of high interest are titanium blades for the compressor section. These airfoils characteristically have a very thin edge and tend to be cambered; that is, they have a curved cross section, as shown in FIG. 1 herein.
In the prior art processes using small diameter shot impelled by airblasts, it was possible to orient the nozzles in the manner which avoided any severe impact on the edges, while at the same time achieving the desired peening effect. (Of course the prior art processes did not provide the smooth finish which is now desired.) In the processes of the recent inventions, relatively heavy steel shot is allowed to fall by gravity, along an essentially straight vertical path. The airfoil to be peened is placed in the shot streamline, so that it is suitably impacted. As reference to the Figures herein shows there are two principal sides to an airfoil. The logical approach taken initially was to continuously rotate the airfoil during peening. However, when this is done the impact of the shot on the thin edges causes damage to them, as they tend to be rolled over and locally deformed by direct shot impacts. While mechanical masking of the edges may be employed, this not only raises cost, but would prevent the desirable compressive stresses from being imparted to the edges where they are needed for good fatigue life. Similarly, simply disposing the airfoil so that one side is first peened, and then the other, without exposing the edges, won't provide the desired stresses at the edges. Therefore, it has been necessary to develop improved procedures to fully peen an airfoil.